The reproduction of video images can be significantly enhanced if the image information is provided in digital format. However real time digitally coded video signals occupy significantly more bandwidth than analog video signals. In addition the data rates available in present transmission channels are limited to rates nominally insufficient to handle raw digitally encoded video signals. Hence the digital video signals are compressed before transmission.
Compressed video data occurs at variable rates, that is, respective frames include different amounts of compressed data. It is desirable to transmit the compressed data at a constant rate equivalent to the channel capacity to realize efficient use of the channel. Rate buffers are implemented to perform variable to constant rate translation. The amount of data provided to the buffers is regulated in accordance with buffer occupancy. This is effected by monitoring the buffer occupancy to generate control signals. Typically the compressors include apparatus which variably quantize compressed data, according to some function of the control signals, to regulate the amount of compressed data provided to the rate buffers.
A first currently known method, which may be used to calculate the level of quantization required to compress video data to a particular rate, computes a quantization step size which is applied to an entire frame. This method provides a uniformly quantized image over respective frames, but it is difficult to determine quantization levels which provide data at precisely the desired rate. A second method divides each frame into blocks and generates quantization levels on a block basis. This method more closely provides compressed data at the desired rate, however, because different areas of the image are quantized differently, the image may not be reproduced with uniform quality.
The present invention includes apparatus for quantizing video signal on a block by block basis to provide data at a relatively constant rate, and which tends not to introduce non-uniformities in image quality.